Julia, The main part of the hat is made up of 8 petal shaped pieces that you sew together. If you took the circumference of your son’s head and divided it by 8, you’d have the bottom width of the petal shaped piece. It shouldn’t be too hard to redraw the petal shaped piece in the adult size using the child size as a guide. Because it’s divided among 8 pieces, there probably won’t be a huge difference in size between the child size piece and the adult size piece. Then, you’d need to adjust the brim piece the same way. I’d make it out of a muslin first so you could make any adjustments in the pattern before making it out of the good fabric. –Anne